In the production of seamless tubing, metal blanks are initially passed through one or more stages of piercing mills, in which an initially solid workpiece is driven spirally over a piercing point, to form a hollow tubular shell. After the workpiece has passed through the piercing mill, the mandrel, which supports the piercing point during the piercing operations, is withdrawn from the processed shell, and the shell is subsequently processed in the plug mill. In a conventional plug mill system, the tubular shell is driven through the mill stand, over a cylindrical plug, reducing the side wall thickness thereof and increasing tube length. Conventionally, two passes through the plug mill are required, with the tubular shell being rotated 90.degree. from its original orientation in preparation for the second pass through the mill.
In a conventional plug mill, the mandrel plug is supported on the end of an elongated mandrel bar, which is in compression during the rolling operation. As soon as the tubular shell passes completely through the mill rolls, and its trailing end is beyond the mandrel plug, the plug is removed. The mill rolls are then opened slightly, and the shell is gripped on the downstream side of the mill rolls by reversely rotating stripper rolls, which return the shell back to the upstream side of the primary mill rolls. When the shell is back on the upstream side of the mill, a new mandrel plug is brought into position (and mill rolls closed) and the shell, now rotated 90.degree. from its original orientation, is sent back through the mill for its second pass. At the end of the second pass, the same return sequence is repeated, with the just-used plug being removed, the primary mill rolls opening slightly, and the stripper rolls driving the processed tubular shell back to the upstream side of the mill rolls. After its second pass, through the plug mill, the tubular shell is removed for further processing, such as in a reeling mill.
As will be appreciated, there is considerable non-productive time involved in a conventional plug mill sequence, while the mill rolls are opened up and the shell is returned back to the upstream side of the mill. Even though the shell is returned at relatively high speeds, the time loss is significant to the point that, in a conventional mill operation, the plug mill may constitute a bottleneck in the overall production sequence.
In accordance with the present invention, an improved plug mill system and method of operation is provided which avoids the need for stripping of the tubular shell off a mandrel, as an independent operation after each pass through the plug mill. Rather, in accordance with the invention, a system of circulating mandrels is provided, in conjunction with a pair of sequentially related mill stands. The arrangement is such that the operation of stripping the tubular shell off a mandrel is accomplished in conjunction with a working pass of the tubular shell through a mill stand, rather than as an independent operation. This enables the shell to receive the necessary two passes through the plug mill stands in substantially less time than in a conventional mill, to the extent that the plug mill operation is no longer a bottleneck in the production sequence.
While the method and apparatus of the present invention involves the use of two sequentially related mill stands, whereas only a single mill stand is used in a conventional mill, the increase in capital costs is relatively limited and very favorable in terms of the increase in performance achieved. In this respect, the mill stands can be much simpler in design than conventional mills, inasmuch as no provision is necessary for quick opening of the rolls. Likewise, it is not necessary to provide stripping rolls, and the related drive systems therefor, for returning the shell to the upstream side of the mill stands. In addition, although a circulating supply of mandrels is provided in the system of the invention, it is not necessary to provide the somewhat more complex plug changing systems typically required in conventional systems, in an effort to minimize cycle time. Moreover, in some embodiments of the new system, it is possible to utilize the mandrels in tension, rather than in compression as is customary in conventional plug mills, enabling smaller mandrels to be utilized.
For a better understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of preferred embodiments of the invention, and to the accompanying drawings.